Direct drive hub motor for 1/10 car - outrunner with sensors? FOC? Needs reliable start from stop.

[GordonFreeman]

Years ago I experimented with a direct drive hub motor for small 1/10 RC cars. It seemed promising except for start up problems. I decided to revisit this idea and sort of decided that I was going to just have to "bite the bullet" and put a hall sensor board in a motor. So I have an old original brushless Novak motor laying around and I thought I would just try to replicate that but for a different outrunner motor. The Novak style motor is extremely common for 1/10 size cars and this sensored style of motor is the only thing used for 1/10 racing.

I have seen the FOC stuff that appears to be for "crawling". Now I like this idea, but why is it only for crawling? Would it not work fine for racing too? Or is it not good at higher speeds? I've seen one ESC that says it uses FOC startup and switches to BEMF mode once it's got some RPM. I don't like the idea of only having one choice for a ESC though.

I'm leaning towards a sensored ESC since there are lots of choices for that. Ideally it should be small and only needs to be capable of roughly 25% the power of 540 motor for a 4wd car.

It seems that in the 1/10 car world that most (if not all) sensored motors will work fine with most (if not all) sensored ESCs. So I would assume that the sensor boards all use the same or at least the same spec hall sensors and the ESC expect the same signal from any given motor.

Anyway, the old Novak motor has hall sensors marked with "3231". I've found this item on Digikey or where ever. Now I don't think I want to deal with Surface Mount, I can however assemble through hole stuff ok. What specs should I be trying to compare so I get a compatible through hole hall sensor?

The outrunner I have in mind is a 2814, 12 slot, 7 mag poles (14 magnets). I figure the easiest thing to do is place the hall sensors at 120 degrees, since the board is going to be fairly small.

I cut excess material off the back side of the motor to fit a sensor board in. I have it sitting on standoffs that will slide/rotate in the existing slots in the back of the motor to fine tune it's location. Some pics attached for your amusement.

Thoughts? Mostly on how to make sure I'm picking the right hall sensor.

 

[neptronix]

You could probably run it sensorless with a VESC just fine.
Those controllers will run anything.

 

[amberwolf]

I have seen the FOC stuff that appears to be for "crawling". Now I like this idea, but why is it only for crawling? Would it not work fine for racing too? Or is it not good at higher speeds?

I don't know what crawling refers to, it's not a term I've seen for any controllers discussed here so far.

I've seen one ESC that says it uses FOC startup and switches to BEMF mode once it's got some RPM.

I think they probably mean sensored startup and sensorless (BEMF) run. FOC would be used for the entire control process, or not at all, AFAIK.

I don't like the idea of only having one choice for a ESC though.

Dunno about controllers for small stuff, but there's plenty of ebike, scooter, motorcycle, etc controllers that do FOC.

Some require the hall sensors for startup, then transition to sensorless once it's moving, becuase it is difficult for most of these systems to tell where the rotor is relative to stator until it's moving, but position sensors of whatever type can provide that info even from a stop.

The smallest FOC I know of that can do complete sensorless operation is the Phaserunner or Baserunner or Frankenrunner by ebikes.ca. I use two PRs to run my heavy heavy-cargo trike SB Cruiser, and each can be held in the palm of my hand.


VESC has many variations for all sorts of applications, but I don't know the specifics on any of them regarding complete sensorless operation--you'd have to lookup the ones you're interested in. There's threads here about the reliablity of variosu manufacturers, and Trampa seems to be among the best, and flipsky among the worst listed by name.


If you have a reliable powerstage to wire it to, the Lebowski brain does FOC, but I think it requires halls for startup. There is a separate thread from the brain board thread for an idea that lebowski tested for reliable sensorless tartup; perhaps you can combine the code from them to make a brain board that doesnt' need sensors.


The one catch with any of these controllers is that if you have a very fast RPM motor, especially if there are a lot of poles, you may run into ERPM limits of the ocntorller above a certain speed, and have unrelaible operation above that point.

It seems that in the 1/10 car world that most (if not all) sensored motors will work fine with most (if not all) sensored ESCs. So I would assume that the sensor boards all use the same or at least the same spec hall sensors and the ESC expect the same signal from any given motor.

I don't know about the RC motors, but with most ebike, scooter controllers, they use UVW / ABC phase / hall sensors.

Larger systems like motorcycles and cars often use different encoders, usually not mounted in the stator teeth like most of the ebike/scooter mtoors (which is a terripble place for them for a number of reasons, but it's really cheap to do).

Slow high torque motors often use SIN/COS encoders for more accurate positioning. (powerchairs, etc)

Most controllers can only read one kind of sensor, so you have to make sure the one you pick is the same type your motor has.

Anyway, the old Novak motor has hall sensors marked with "3231". I've found this item on Digikey or where ever. Now I don't think I want to deal with Surface Mount, I can however assemble through hole stuff ok. What specs should I be trying to compare so I get a compatible through hole hall sensor?

You'd have to find out what kind of signal your controller expects, and use a part compatible with that. There are many types with differnet output operation.

With ebike / scooter controllers, almost all expect a signal from an open-collector latching hall sensor, which simply grounds the signal provided on the hall sensor line from the controller, whenever the polarity of the magnetic field it detects changes. These almost always use 4.5-5v to power the sensors, and have a 5v pullup on the signal wire, so most commonly the honeywell / allegro SS41 or SS411 is used, with whichever letter designation matches the voltage and temperature range being used. (or some unbranded clone of one of these sensors). See the spec sheets for those sensors on mouser/digikey/etc for whihc version matches your needs.

 

[GordonFreeman]

Thanks for the input so far.

"Crawling" refers to rock crawling. Some RCers like the scale looks and technical obstacle courses. But I refer to "crawling" because part of it is going very slow and have good wheel speed/torque control. Now most of these rigs are NOT direct drive, so that may still be an issue for me.

My experience trying to do this awhile back was that normal BEMF sensorless brushless control is nowhere good enough to reliably start a direct drive wheel motor. You have to futz and poke at the throttle until it randomly started in the direction you wanted. And be aware that if you do it wrong, the esc may burn up.

I had/have a VESC, but unfortunately the last time I tried to hook it up to my computer I couldn't get it to work. I had tried some foc with it back then and was impressed. If I could find one small enough, I'd probably try it, but the market for those is for much bigger projects I think.

Preferably I'd like an esc that costs roughly 1/4 of what a normal 1/10 esc does, since I'd be using 4 of them. Size should be half the size of a normal 1/10 esc. But since I have sensored escs laying around I thought I'd give that a shot with a custom sensor board on an outrunner I also have laying around.

Furitek looks to have some interesting options. The local mini-z track has some cars that I believe are sensorless and they seem to start up just fine, again, they are not direct drive though.